Modern aluminum alloys for high strength application are strengthened by solution heat treatment and fast cooling followed by an age hardening process. Rapid cooling is commonly achieved by cold water quench. Without such a fast quench process immediately after the solution heat treatment, the age hardening process becomes very ineffective.
The fast cooling process is usually carried out by rapid heat transfer into cold water, which has a high heat capacity. However, the internal volume of thick gauge wrought products cannot be quenched sufficiently fast due to slow heat transfer through the thickness of the product. Therefore, an aluminum alloy suitable for very thick gauge product is needed. Such an alloy should be able to maintain good age hardening capability even after a relatively slow quench process.
Fast cooling by cold-water quench has the serious drawback, however, of raising internal residual stress, which is detrimental to machinability. The most common practice to reduce such residual stress is to cold stretch the quenched product by a small amount typically by using a stretcher machine. As the thickness and width of wrought product increases, the force required to stretch such a product increases. In consequence, a powerful stretcher is necessary as the product dimension increases such that the stretcher becomes the limiting factor in deciding the maximum wrought product thickness and width.
The stretcher can be eliminated as a limiting factor if the wrought product can be slow cooled without a cold-water quench after solution treatment. Thus, residual stress would be minimal and cold stretching would not be required.
The desirable high strength aluminum alloy most suitable for ultra thick gauge wrought product should therefore be capable of achieving desirable high strength in age strengthened temper after solution heat treatment followed by a relatively slow quench.